1. Field of the Invention
The present invention relates to an automatic gain control circuit in a radio data communication system using a spread spectrum method, and in particular, an automatic gain control circuit which is digitally embodied to prevent deterioration of a bit error rate generated by saturation of a low noise amplifier located at the front stage of a receiver through an input of high power to an antenna, thereby simplifying construction thereof and improving commercialization.
2. Description of the Related Art
In general, a spread spectrum method is one type of communication method to transmit digital data in buildings. Thus, the general spread spectrum modulation method is to produce a radio frequency (RF) having high frequency and pseudo-noise (PN) coded digital data, through use of a mixer, as a spread signal having wide bandwidth and low spectrum density. As a result, the spread spectrum modulation method has a feature capable of improving performance of a system due to a decrease in external interference on the system. In order to receive the spread signal formed and transmitted by the spread spectrum method and include a stable demodulation of the received signal, it is necessary to maintain a constant level of the received signal. Therefore, an automatic gain control (AGC) amplifier located at the front stage of a receiver is utilized to maintain the received signal at a constant level.
Radio receivers having an automatic gain control (AGC) circuit have various forms. An example of a radio receiver having an AGC circuit is one comprised of: first and second signal mixers for down converting a received spread signal utilizing output frequencies of a local oscillator into an intermediate frequency (IF) signal; an IF amplifier for amplifying the IF signal and for generating a voltage according to a received signal strength indicator (RSSI); a voltage controlled oscillator for generating a reference frequency; a phase shifter for shifting the phase of the reference frequency by 90.degree.; an I-channel mixer for mixing the amplified IF signal output of the IF amplifier with the reference frequency; a Q-channel mixer for mixing the amplified IF signal output of the IF amplifier with the phase shifted reference frequency; low pass filters for eliminating a low frequency signal from the I and Q channel signals output from the I and Q channel mixers; first and second amplifiers for amplifying the filtered signals output from the low pass filters; analog/digital (A/D) converters for converting the amplified signals output by the first and second amplifiers into respective digital signals; a modem for demodulating data from the A/D converted signals of the I and Q channels; a low pass filter for filtering the RSSI voltage; a level converter for converting the level of the signal output from the low pass filter; and a log amplifier for converting the level-converted signal in conformity with a narrow scope of a control voltage of the AGC amplifier.
As constructed above, the radio receiver uses the IF amplifier to control the AGC amplifier with the voltage proportioned to the level of the received signal. However, it is generally improper to use the RSSI voltage output of the IF amplifier directly in the above case. Therefore, before filtering the level of the RSSI voltage in the low pass filter, the level converter is necessarily required for converting the level of the RSSI voltage in conformity with the scope of the control voltage desired by the AGC amplifier. Besides, if the level of the voltage having the proper scope can not be gained for controlling the AGC amplifier even after the level of the RSSI voltage is converted, that is, if the dynamic area of the RSSI is wide while that of the control voltage of the AGC amplifier is very narrow, the radio receiver generates the control voltage having the proper scope through decreasing variation of output level with respect to input level due to use of the log amplifier.
As mentioned above, the automatic gain control method using the level converter or the log amplifier is not easy to embody the level converter, and the embodiment of the level converter may be frequently impossible in accordance with the scope of variable control voltage of the AGC amplifier. At this time, the level converter must be embodied by inevitably adding another separate circuit. Besides, as operation frequency becomes higher, it has a difficulty in embodying the log amplifier needed when the dynamic area of the AGC is very narrow and the dynamic area of the RSSI voltage as the output of the IF amplifier is wide. Moreover, the bandwidth of the low pass filter determines the feature of response time of an automatic gain control loop. If the bandwidth of the low pass filter becomes wide, the feature of the response time of the automatic gain control loop is improved while noise level in the loop is increased. As a result, gain variation of the AGC amplifier naturally becomes excessive because the control voltage of the AGC amplifier varies too rapidly. On the other hand, if the bandwidth of the LPF becomes small, the gain variation of the AGC amplifier relatively reduces more than the above case due to a decrease of the noise level in the loop, however, the feature of the response time of the automatic gain control loop may be deteriorated. Finally, to solve the above described problem, it is required to properly select the bandwidth of the LPF in order to have no effect on the performance of apparatus. However, this selection is not so easy.